In recent years there has been considerable interest in building low-loss optical waveguides on various planar substrates. These waveguides are needed for numerous devices such as directional couplers, filters, switches, and optical interconnections for electronic circuits, to name just a few.
The tremendous technical progress in electronic devices built on silicon and gallium arsenide substrates has given rise to the need for constructing optical waveguides on these materials. Gallium arsenide devices such as lasers and FET's have been built on silicon wafers, so that the possibility now exists of interconnecting silicon electronic circuits with optical waveguides, provided these can be built with a sufficiently low loss. The extremely high bandwidth of optical waveguides would allow one to build processing systems operating at rates of tens of gigabits per second, opening the way to important applications in communications and computers.
Much effort has been directed at constructing optical waveguides on silicon wafers because they are mechanically sturdy and readily lend themselves to integration with silicon electronic circuits. A common approach has been to first grow a layer of silicon dioxide (SiO.sub.2) by thermal oxidation of the silicon substrate (to be referred to hereafter as "thermal SiO.sub.2 "), and then to form on top of the thermal SiO.sub.2 a layer of SiO.sub.2 doped with elements that cause a slight increase .DELTA.n in the refractive index relative to that of pure SiO.sub.2. Another approach has been to deposit on top of the thermal SiO.sub.2 a layer of silicon nitride (Si.sub.3 N.sub.4) or of titanium dioxide (TiO.sub.2), which have refractive indexes of 2.1 and 1.7 respectively. In these waveguides optical power is partially confined to the higher index layer by the standard process of total internal reflection at the interface between the two layers.
Two problems that have been encountered with previous approaches are that the doped SiO.sub.2 or the deposited Si.sub.3 N.sub.4 or TiO.sub.2 are more lossy than pure SiO.sub.2, and optical confinement is often weak leading to a strong evanescent field at the silicon substrate. Because silicon has a high refractive index (approximately 3.5), the evanescent field becomes a propagating wave in the silicon substrate and causes a substantial loss of power by radiation into the substrate.
Various possibilities exist to minimize this radiative loss. One approach is to use a thick layer of the high-index material, like Si.sub.3 N.sub.4 or TiO.sub.2, in order to better confine the optical power to the top layer. But this often leads to higher losses and to deleterious higher order modes, and also restricts the mode size to small dimensions which poorly couple to optical fibers.
In another approach, an extremely thick layer of SiO.sub.2 is grown over the silicon substrate by thermal oxidation; this thick oxide layer keeps the silicon substrate as far out as possible on the tail of the evanescent field, thus reducing the "leakage" of optical power into the substrate. But this extra-thick oxide may require several days, sometimes even weeks, of growth time to achieve the desired degree of isolation.